1) Long Term Evolution Device to Network (LTE D2N) Communication System
As illustrated in FIG. 1, in the LTE system, D2N communication is controlled centrally by the network, that is, both uplink and downlink data of a User Equipment (UE) (also referred to a terminal) are transmitted and received under the control of the network. Communication between one UE and another UE is forwarded and controlled by the network, and there is no direct communication link between the UE and the other UE, where data transmission between the UE and the network in this mode can be referred simply to as D2N transmission.
2) Device to Device (D2D) Proximity Services
As illustrated in FIG. 2, the D2D proximity services are generally categorized as follows in the 3rd Generation Partnership Project (3GPP):
D2D proximity service discovery (D2D Discovery): A UE determines another UE proximate thereto over an Evolved Universal Terrestrial Radio Access Network (E-UTRAN). For example, the D2D UE can search for a taxi proximate thereto, a friend proximate thereto, etc., using this service.
D2D proximity service communication (D2D Communication): A link is set up directly between two UEs proximate to each other, so that a communication link originally for transmission over the network is translated into a local direct communication link to thereby significantly save a bandwidth and improve the efficiency of the network; or two UEs proximate to each other can be provided with a high-speed and cheap communication service by communicating over a direct link. Proximity service communication is generally conducted under the control or assistance of the network side, where an evolved Node B (eNB) (also referred to a base station) even allocate dynamically a resource for a UE in proximity service communication.
For the sake of a convenient description, two types of links can be defined as follows:
A D2D link refers to a link for direct communication between one device and another device; and
A D2N link refers to a link for communication between a device and a network node.
Moreover the UEs participating D2D Discovery and/or Communication play two roles as follows:
A D2D transmitting UE refers to a UE transmitting a D2D Discovery and/or Communication message; and
A D2D receiving UE refers to a UE receiving the D2D Discovery and/or Communication message transmitted by the D2D transmitting UE.
3) Allocation Types of D2D Discovery Resources
There are two allocation types of D2D Discovery resources currently defined in the 3GPP as follows:
Type1 where the UE selects one of resources in a pool of transmission resources to transmit a D2D Discovery message, i.e., a UE selection mode; and
Type2 where the UE transmits a D2D Discovery message over a resource allocated by the network, i.e., a network scheduling mode. Type2 can be further categorized into two allocation types of resources, i.e., Type2A and Type2B, dependent upon whether the D2D Discovery message transmission resource allocated by the network is dynamic or semi-static. For Type2A, the network may allocate dynamically a resource for the D2D Discovery transmitting UE to transmit a D2D Discovery message; and for Type2B, the network may allocate semi-statically a resource for the D2D Discovery transmitting UE to transmit a D2D Discovery message.
4) Allocation Types of D2D Communication Resources
There are also two allocation types of D2D Communication resources currently defined in the 3GPP as follows:
Mode1 where the network schedules a D2D transmission resource for the D2D Communication transmitting UE, i.e., a network scheduling mode; and
Mode2 where the UE selects one of resources in a pool of D2D Communication transmission resources to transmit a D2D Communication message, i.e., a UE selection mode.
If the allocation type of the D2D Discovery and/or Communication resource selected by the D2D transmitting UE is controlled by the network, then the D2D transmitting UE in the D2N connected state may determine a D2D transmission resource to be used in a destination cell when a cell handover occurs.